Aqueous emulsions of N-methylol acrylamide--vinyl acetate polymers are widely used in formulating wood adhesives wherein bond strength, fast setting, colorless glue lines and ease of application are desired. Protective colloids are often added to stabilize the emulsions against gelling during storage and to improve water resistance. For these purposes, 88% hydrolyzed polyvinyl alcohol is the most commonly used protective colloid. It is generally agreed that as the degree of hydrolysis of the polyvinyl alcohol increases above 88% its ability to perform as an effective protective colloid decreases leading to lower mechanical stability of the final latex. This problem is accentuated by the presence of water soluble resins such as resorcinol-formaldehyde, urea-formaldehyde, melamine-formaldehyde, or phenol-formaldehyde resins and trimethylol phenol oligomer or with fusion aids, such as tetrahydrofurfuryl alcohol or ethylene glycol diacetate which are generally added to improve water resistance and other properties of these adhesives. These pre-formulated adhesives may then be catalyzed with various acidic metal salts as is known in the art. These adhesive systems are described, for example, in U.S. Pat. Nos. 3,301,809; 3,563,851; 3,433,701; 3,041,301; 4,474,915 and 4,687,809.
While such formulated vinyl acetate based adhesives provide adequate water resistance for plywood and finger jointed lumber constructions tested at room temperature, under vacuum-pressure soaks and in boiling water these vinyl acetate adhesives do not meet ASTM standards for exterior lumber, especially ASTM D-3110 for edge glued lumber in general. This is illustrated by the requirements for these tests on lumber, as well as the values obtained using conventional formulated crosslinked polyvinyl acetate adhesives (PVA) on pine presented in the table below.
______________________________________ Room Temp. Pressure Soaks Boils ______________________________________ Finger-Jointed Typical PVA 4498 psi, 2400 psi, 2200 psi, (Pine) 96% WF 68% WF 60% WF Requirements 2000 psi, 1600 psi, 1600 psi, 60% WF 50% WF 50% WF Edge Glued Typical PVA 1411 psi, 275 psi, 300 psi, (Pine) 97% WF 20% WF 15% WF Requirements 678 psi, 565 psi, 565 psi, 60% WF 50% WF 50% WF ______________________________________ WF = % wood failure or % wood tear